Computer systems typically have the integrated circuits (ICs) interconnected on a motherboard. These ICs send signals between various circuit functions using drivers and receivers. On-chip circuits use drivers and receivers configured for the on-chip transmission lines which are typically very short but highly resistive and lossy. Off-chip drivers are used for signals transmitted between ICs. As off-chip communication speeds have increased, the lines interconnecting the ICs should be treated as transmission lines when their lengths are long relative to the fundamental wavelengths of the signals they transmit. Off-chip transmission lines are typically low-loss with characteristic impedances typically between 50 and 70 ohms.
Off-chip drivers that have signal swings extending to the power supply rails require high currents relative to the currents for on-chip driver circuits. As off-chip driver speeds have increased so has the number of bits for the data buses used in off-chip communication. Most logic in the various ICs making up a computer system are synchronous, wherein a system clock is used to time when data is transmitted or applied to one end of a transmission line and when it is sampled by the receiver at the other end of the transmission line.
Various types of noise may result from off-chip driving including simultaneous switching noise, electric and magnetic field coupling between signal transmission lines, etc. Likewise, the power supply noise resulting from large current spikes during simultaneous switching in an IC transmitting signals may be different from the power supply noise present at an IC receiving the signals. As system clocking speeds have increased, the power supply voltages have been decreased to manage power. The higher speed operation along with lower power supply voltages may lead to decreased noise margins. Some of the noise experienced in off-chip communication may be common mode where the noise appears simultaneously on both the voltage and ground lines (planes) of the motherboard. To improve the noise rejection when receiving off-chip signals, differential receivers are often used where the receivers are biased at a reference level (e.g., one half the power supply voltage).
It is also advantageous to terminate the transmission lines interconnecting ICs in the characteristic impedance of the transmission line. While the transmission line may be series or source terminated, far end termination often provides the best overall performance at the expense of power dissipation especially when the transmission line network has multi-drop nets (e.g., a particular bit is coupled to more than one IC). All the above factors create a signal transmission environment which is difficult to optimize so as to ensure the best possible noise margins.
There is, therefore, a need for signal transmission circuitry and a method for optimizing the noise margins in high speed digital signal transmission and reception.